diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2832560a3..dfce8e10d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@
 - fix calling return type getter function `@Field(type => Foo)` before finishing module evaluation (allow for extending circular classes using `require`)
 - fix nullifying other custom method decorators - call the method on target instance, not the stored reference to original function (#247)
 - fix throwing error when extending non args class in the `@ArgsType()` class
+- prevent unnecessary conversion of an object that is already an instance of the requested type (avoid constructor side-effects)
 
 ## v0.16.0
 ### Features
diff --git a/src/helpers/types.ts b/src/helpers/types.ts
index 2aa6385c1..13713c423 100644
--- a/src/helpers/types.ts
+++ b/src/helpers/types.ts
@@ -90,6 +90,10 @@ export function convertToType(Target: any, data?: object): object | undefined {
   if (simpleTypes.includes(data.constructor)) {
     return data;
   }
+  // skip converting already converted types
+  if (data instanceof Target) {
+    return data;
+  }
 
   return Object.assign(new Target(), data);
 }
diff --git a/tests/functional/resolvers.ts b/tests/functional/resolvers.ts
index bdf2fc1f7..5abee89cf 100644
--- a/tests/functional/resolvers.ts
+++ b/tests/functional/resolvers.ts
@@ -1083,6 +1083,7 @@ describe("Resolvers", () => {
     let queryContext: any;
     let queryInfo: any;
     let descriptorEvaluated: boolean;
+    let sampleObjectConstructorCallCount: number;
 
     function DescriptorDecorator(): MethodDecorator {
       return (obj, methodName, descriptor: any) => {
@@ -1119,6 +1120,7 @@ describe("Resolvers", () => {
       queryContext = undefined;
       queryInfo = undefined;
       descriptorEvaluated = false;
+      sampleObjectConstructorCallCount = 0;
     });
 
     beforeAll(async () => {
@@ -1156,6 +1158,9 @@ describe("Resolvers", () => {
         isTrue() {
           return this.TRUE;
         }
+        constructor() {
+          sampleObjectConstructorCallCount++;
+        }
 
         instanceValue = Math.random();
 
@@ -1427,6 +1432,24 @@ describe("Resolvers", () => {
       expect(getterFieldResult1).not.toEqual(getterFieldResult2);
     });
 
+    it("shouldn't create new instance for object type if it's already an instance of its class", async () => {
+      const query = /* graphql */ `
+        query {
+          sampleQuery {
+            getterField
+            methodField
+          }
+        }
+      `;
+
+      const result = await graphql(schema, query);
+      const getterFieldValue = result.data!.sampleQuery.getterField;
+      const methodFieldValue = result.data!.sampleQuery.getterField;
+
+      expect(getterFieldValue).toEqual(methodFieldValue);
+      expect(sampleObjectConstructorCallCount).toBe(1);
+    });
+
     it("should use the same instance of resolver class for consecutive queries", async () => {
       const query = `query {
         sampleQuery {